Thermal barrier coating systems for components, such as blades and vanes, used in gas turbine engines are the subject of numerous patents including, but not limited to, U.S. Pat. Nos. 4,405,659; 4,676,994; 5,015,502; and 5,514,482. Thermal barrier coating systems typically are applied on a superalloy substrate and involve an inner bondcoat on the substrate, an outer thermal insulating layer which typically comprises one or more ceramic materials, and a thin intermediate ceramic layer located between the bondcoat and the thermally insulating layer to promote adherence of the thermally insulating layer. A typical TBC coating system uses an alumina (aluminum oxide) intermediate layer between a ceramic (e.g. zirconia stablized with yttria) thermal insulating layer and a metallic and/or diffusion aluminide bondcoat.
In such TBC coating systems, spallation of the outer thermal insulating layer can occur at the intermediate layer as a result of residual compressive stresses in the TBC system, especially between the bond coat and intermediate layer. This is especially true of TBC systems where the thermal insulating layer is electron beam evaporated and physical vapor deposited (EB-PVD) on an intermediate ceramic layer, such as alumina. The influence of residual compressive stresses in thermal barrier coating (TBC) systems is closely linked to TBC adherence properties observed in testing and service.
Residual stress measurement techniques such as X-ray diffraction have been of limited use in determining residual compressive stress of TBC systems due in large part to the difficulty in penetrating through the thermal insulating layer to the intermediate layer. The intermediate layer also is very thin (e.g. 1 micron thickness) and is therefore very difficult to characterize by X-ray diffraction and other conventional techniques such as neutron diffraction.
An object of the present invention is to provide a non-destructive measurement method for determining residual stress proximate an intermediate layer in a multilayer a TBC coating system.
Another object is to provide a a quality control procedure or repair that uses such residual stress measurement as a means for determining acceptability of a manufactured TBC coating system for service or of an engine-run TBC coating for return to service during inspection or repair procedures.